<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>4.1</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/css/bootstrap.min.css">
    <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>
    <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/js/bootstrap.min.js"></script>
</head>
<style>
    .breadcrumb-item +.breadcrumb-item::before{
        display: inline-block;
        padding-right: 0.5rem;
        color: #6c757d;
        content: ">";
    }
</style>
<body class="container">
    <h3 class="mb-4">面包屑</h3>
    <nav aria-label="breadcrumb">
<ol class="breadcrumb">
<li class="breadcrumb-item active">首页</li>
</ol>
    </nav>
    <nav aria-label="breadcrumb">
        <ol class="breadcrumb">
        <li class="breadcrumb-item active"><a href="#">首页</a></li>
        <li class="breadcrumb-item ">图书馆</li>
        </ol>
            </nav>
            <nav aria-label="breadcrumb">
                <ol class="breadcrumb">
                <li class="breadcrumb-item "><a href="#">首页</a></li>
                <li class="breadcrumb-item "><a href="#">图书馆</a></li>
                <li class="breadcrumb-item active">工程类</li>
                </ol>
                    </nav>
            <br>
</body>
</html>